1. Field of Invention
The present invention relates to a process for treating a surface of a perforated mask made of iron. The process provides a grayish black coating having an excellent thermal emission power on the mask by a predetermined heat treatment.
2. Description of Prior Art
A perforated mask used as a color-selecting electrode of a color picture tube such as a black matrix type color picture tube has been almost made of iron recently and, in some cases, manufactured by forming fluorescent dots or fluorescent stripes and then expanding pores for electron beams by the post-etching technique.
A post-etched perforated mask has a surface of metal exposed. Therefore, if any surface treatment is not effected to such mask, red rust is partially produced by a post heat treatment. This rust is peeled and adheres to the perforated mask. Thus, some pores of the mask are blocked. And some of electron beams are lacked. Further, the temperature of the surface of the perforated mask is raised by scanning the mask with electron beams, because of a poor thermal emission power of the mask after heat treating and, therefore, reversible thermal expansion of the mask occurs. As a result, electron beams have a great tendency to deviate from the corresponding fluorescent elements and inconveniently produce non-uniformity of color.
Therefore, for improving a thermal emission power there has been heretofore employed a process in which a black film of ferrosoferric oxide is formed on the surface of the mask by the chemically blackening method. The rust proofing as well as the thermal emission power is improved by the process.
This process is carried out by using an about 50% solution of caustic soda and at a temperature of about 140.degree.C. When a mask to be treated is dipped into the solution, the difference in thermal expansion between a frame member and a perforated sheet member of the mask occurs due to the difference in heat capacity between the two members. Thus, permanent deformation like wrinkles are produced on the perforated sheet member. These wrinkles cause a non-uniformity of color.
This process uses a strong alkaline solution at an elevated temperature and, therefore, is accompanied with a great danger and further requires a waste-water treating system having a large scale.